1. Field of the Invention
The present invention relates to a developing unit incorporated into an image forming apparatus such as an electronic copying machine.
2. Description of the Related Art
A conventional developing unit includes a developer container for containing developers, a rotatable development roller, and a rotatable mixer. The development roller has, for example, a rotational development sleeve and a magnet having a number of magnetic poles arranged in the development sleeve in the rotating direction of the development sleeve.
The developing unit performs development as follows. First the mixer is rotated, and so is the development sleeve. In accordance with the rotation of the mixer, developers more than a suitable quantity are supplied to the surface of the development sleeve and then carried by rotation of the development sleeve. The quantity of the developers is restricted to the suitable quantity by a doctor blade during the carriage of the developers. The suitable quantity of developers are then supplied to a development position close to a photosensitive drum, thereby developing an electrostatic latent image.
In the development, not all the developers are supplied to the electrostatic latent image from the development sleeve, but some of them remain thereon. If the remaining developers are not collected in the mixer but recycled as they are, image density is lowered and image quality is degraded. In order to eliminate this problem, in the conventional developing unit, adjacent magnetic poles of the magnet, which are located on the opposite side of the development sleeve facing the photosensitive drum, are set to the same, and the developers, which have contributed to the development or passed the development position, are peeled from the development sleeve by magnetic force between the same poles and then collected in the mixer.
Since, however, the developers are not peeled sufficiently by the magnetic force between the adjacent magnetic poles, a large number of developers remain on the development sleeve. In the conventional developing unit, image quality is prevented from deteriorating by taking into consideration an angle between the adjacent magnetic poles of the magnet or a balance of the magnetic force, or by regulating the quantity of developers in the developer container to prevent the peeled developers from being attached to the development sleeve again.
It is however difficult to suitably set the angle between the adjacent magnetic poles or the balance of the magnetic force because a relationship between the magnetic poles of the magnet has to be considered.
The developing unit disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-108087 comprises a scratch member formed of a flexible member such as Mylar which contacts a development sleeve to scratch the residual developers from the development sleeve. However, the contact of the scratch member and development sleeve causes the development sleeve to be damaged and the developers to be crashed. In this conventional developing unit, the developers scratched from the development sleeve remain in the vicinity of the scratch member and are not collected efficiently in a mixer. For this reason, the developers scratched from the development sleeve are returned to the development sleeve again before the mixer, and no fresh developers can be supplied to the development sleeve.